How useful do you find Durkheim’s notion of a “social fact”? Does he do an adequate job of defending this concept, or does it still overlap with other disciplines? Or, is it even possible to single out “social facts” from say, economic or psychological ones? Why or why not?

Durkheim's concept of a "social fact" is a tremendously powerful one, and frankly one I wish more people talked about. The core notion is that social facts are those which don't exist in any particular individual, nor in any particular physical object, but are created by the aggregate behavior of whole societies. Social facts can in some sense be real facts that control our actions, despite not existing in any particular place at any particular time. Durkheim's goal was to carve out a particular space for sociology as distinct from other disciplines, but the idea of a social fact has wider applicability, essentially to all the social sciences. "The British pound has recently been devalued relative to the US dollar" is a social fact that is of interest to economists; "herders typically establish norms of honor-based violence" is a social fact that is of interest to anthropologists. It may not be possible to really separate social facts from psychological facts or economic facts, but this does not mean the concept is useless, only that it doesn't quite achieve the hard-edged distinction between different disciplines that Durkheim himself was aiming for. (Where does psychology end and sociology begin? Well, where does physics end and chemistry begin? Does it matter?) Social facts can be distinguished on a fundamental level from both physical facts on the one hand and fictions on the other.Physical facts just are. A particular configuration of matter exists in space, and it's there whether or not anyone believes it. "The Earth's mass is 6*10^24 kilograms" is a physical fact.Fictions just are not. They make claims about the universe that simply fail to hold, again whether or not anyone believes them. "Harry Potter is a wizard" is a fiction.But social facts are if we believe they are, or more precisely are if a critical mass in society acts as though they are. "The British pound is worth more than the US dollar" is a social fact. The British pound and the US dollar have no inherent value; they are slips of paper or numbers on a spreadsheet. But they take on value when we use them as though they have value, and that is not purely a fiction. It is not imaginary that a British pound will buy you more things than a US dollar---it really will buy you more things. But that is only true because millions of people in the US and Britain believe and act as though it is true, and if they ever started acting differently, it wouldn't be true anymore.
https://web.archive.org/web/20121003050946/http://www.faculty.rsu.edu/users/f/felwell/www/TheoryWeb/readings/DurkheimFactForm.html